Got ornaments, rocks, and other things covered in algae that you want cleaned? If so, there are several methods available for cleaning them. Mostly it comes down to how much time, effort, and money you want to spend. It also depends on what needs cleaning.
The least time consuming algae removal method is to bleach items using chlorine bleach. First, be sure that the items you intend to clean can be safely exposed to chlorine bleach. Obviously, remove any items that you want cleaned from the aquarium before attempting to clean them with this method. DO NOT ADD BLEACH TO AN AQUARIUM CONTAINING LIVESTOCK. Using a ratio of 1:15 (one part bleach to 15 parts water) should be more than sufficient to kill any type of algae. Use extreme caution when working with chlorine bleach for any reason, including wearing eye protection and gloves.
I won’t go into the details of this process, but you can easily Google them.
Another method that is easy and even cheaper than using chlorine bleach but takes more time is to just remove the objects from the aquarium and place them outside in the sun. This works better when it’s warm outside. Sunny and cold conditions aren’t as ideal, but will work. The problem with this method is that the objects can get insects in them and on them. Also, if the objects are in the sun and aren’t protected from birds, the objects might end up with bird droppings on them. You don’t want to put any object with bird droppings on it into your aquarium. Bird feces may contain parasites. Every time I use the sun bleach method, I boil the items to ensure I don’t transfer any parasites. There are other ways to kill parasites hitching a ride on your aquarium components, but you can Google those.
A third option but the most time and effort consuming is to simply scrub the algae off the items or use a pressure washer on them. Obviously, these two methods depend on the amount of algae, the size of the objects, and of course availability of a pressure washer if you choose to use one. I don’t mind to scrub a small rock or two, but if I have a larger tank with numerous items covered in algae, I usually use the chlorine bleach or sun bleach methods listed above.
For an example of what algae covered holey rock looks like after setting in the sun for a day or so, see this post from 2018.
All methods discussed above can be used on shells also, if you keep shells in your tank. Some of my tanks contain travertine tile, various clay caves, and even PVC. I use these same methods when cleaning these items.